Method of increasing the volume of a permeable zone within an oil shale formation



P. J. CLOSMANN 3,478,25 METHOD OF INCREASING THE VOLUME OF A PERMEABLE Nov. 18. 99

ZONE WITHIN AN OIL SHALE FORMATION 2 Sheets-.Sheet 1 Filed Aug. 2l. 196'? SEPARATOR 3|) HEATERj All AY HEAT ExcHANGERj INVENTOR:

PHULIP J. CLOSMNN HHS ATTORNEY Nov. 18, 1969 P. J. cLosMANN METHOD OF INCREASING THE VOLUME OF A PERMEABLE ZONE WITHIN AN OIL SHALE FORMATION 2 Sheets-Sheet 2 SEPARATOR R E G N A H C X E T A E H FIG. 5

INVENTORZ CLosM'ANN BY: .l y 6 PHILIP J.

HIS ATTORNEY United States Patent O METHOD OF INCREASING THE VOLUME OF A PERMEABLE ZONE WITHIN AN OIL SHALE FORMATION Philip J. Closmann, Houston, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Aug. 21, 1967, Ser. No. 662,110

Int. Cl. E21b 43/26 U.S. Cl. 166-299 10 Claims ABSTRACT F THE DISCLOSURE An improved method of increasing the volume of the permeable zone created by detonation of a relatively high energy device within a subterranean oil shale formation by opening a borehole in the substantially void space created"by the detonation of the device, opening perforations in the fractured zone surrounding `the void space and circulating fluid between the void space and the rubble resulting from the detonation including flow through the perforations.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to an improved method of increasing the volume of a permeable zone formed within an oil shale formation, and, more particularly, to a method of extending channels into the fractured zone surrounding the chimney of rubble created by detonation 'within the oil shale formation of a high energy device.

Description of the prior art The use of contained nuclear explosions has been proposed in subterranean oil Shale formations in an attempt to break up the oil shale formation so that shale oil can be recovered from the rubbled zone by known techniques, such as in situ retorting.

Experience has shown that when a relatively high energy device, such as a nuclear bomb, is exploded within a subterranean earth formation, an almost spherical cavity filled with hot gases is formed. This cavity expands until the pressure within the cavity equals that of the overburden. On cooling, the roof of the cavity collapses since, generally, it cannot support itself, and a so-called chimney develops. Chimney growth ceases when the rock pile substantially -fills the cavity, or, a stable arch develops. In both cases, a substantially void space is formed below the overburden and above the rubble contained within the chimney. Surrounding the chimney is a fractured zone which results from the shock of the nuclear explosion.

One of the chief uncertainties with regard to the effects of nuclear explositions within a subterranean oil shale formation is the permeability distribution surrounding the cavity and subsequent chimney produced by a detonation. Evidence from prior explosions suggests that permeability of the fragmented zone may drop very rapidly -with distance radially out from the primary nibble zone. A high and uniform permeability is important in order to provide maximum sweep efficiency in any underground hydrocarbon recovery process.

SUMMARY OF THE INVENTION It is an object of this invention to establish suitable communication between fractures in the rocks around a chimney of rubble produced by the detonation of a rela-y tively high energy explosive device and the interstices in -the rubble contained within the chimney.

It is a further object to improve the Sweep efliciency of underground hydrocarbon recovery processes used to 3,478,825 Patented Nov. 18, 1969 ICC extract shale oil from a fragmented oil shale formation by increasing the volume of the permeable zone which is created -within the formation by the explosive device.

Theconcepts of this invention are carried out by irnproving the communication with fractures in the region around the chimney of rubble formed by the detonation of a relatively high energy device. A borehole opening is extended into the substantially void spaceat the top of the chimney and a perforating device is lowered through the borehole -opening and directed against the wall of the fractured zone surrounding the chimney so as to Open gener-ally radial channels into the wall.

Fluid is then circulated by known means between the borehole opening and the rubble within the chimney so as to include the channels into the wall of the fractured zone Vthereby increasing the volume of the permeable zone that is swept by the circulating fluid.

If the permeability of the central rubble zone of the chimney is so high as to effectively Idivert all or most of the circulated fluid along a path confined 'within the rubble, material is preferably injected to form an impermeable layer near the top of the rubble contained within the chimney. Alternatively or additionally, a fluid having a relatively high frictional resistance to flow through the rubble contained in the chimney is used as the circulating fluid.

BRIEF DESCRIPTION OF THE DRAWING FIGURE l is a vertical sectional view of an oil shale formation having a well extending into a permeable zone within the formation;

FIGURE 2 is a vertical sectional view of an oil recovery process applied to the oil shale formation of FIGURE 1; and

FIGURE 3 is a vertical sectional view of an alternate oil recovery process applied to the oil shale formation of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning to the drawing, FIGURE l shows a permeable fragmented zone 11 created within an oil shale formation 12. Zone 11 is a nuclear chimney-type permeable zone created by detonating a relatively high energy explosive device, such as a nuclear bomb, within the formation 12. Such detonation has been discovered to create a permeable chimney-shaped rubble-containing zone 11 having a solidified glassy rock portion 13 at its lower end and a substantially void space 14 at its upper end. The remainder of the chimney comprises a rubble 15 of relatively high permeability. Further information concerning the formation of a chimney by subterranean nuclear detonation can be obtained from the copending application to Prats, Ser. No. 613,600, filed Feb. 2, 1-967.

As can be seen in FIGURE 1, the permeable zone 11 is surrounded by a series of fractures 16. Many of these fractures extend along generally vertical planes and do not communicate with the rubble-filled Zone 15. In order to improve'communication bet-Ween the area of relatively high permeability adjacent the middle or center portion 15a of the rubble 15 and the radially outermost region adjacent fractures 16, the rock around the edges of the permeable zone 11 adjacent the substantially void space 14 is perforated to open generally radial channels into the surrounding rocks. For example, a well 17 is extended into the substantially void space 14 and a perforating device 18, such as a laser beam, explosive jet or bullet-type peiforator or the like, is lowered into well 17 by means of a combined load carrying and electrical signal and/or current transmitting cable 19 coupled to a winch 20. Of course, other means, mechanical or electrical or both,

can be used to selectively raise and lower the perforating device 18 within well 17. For reasons of convenience, the remaining above ground components of well 17 have been omitted in FIGURE 1.

In operation, the perforating device 18 is lowered through well 17 into the substantially void space 14 and moved into position so as to form channels that contact the fractures 16 surrounding space 14. Conventional operating or prime mover means 21, coupled to winch 20 and perforating device 18, are preferably used to remotely control the operation of the perforating device 18. Thus, cable 19 is of suicient strength to raise and lower perforating device 18 and contains therein the necessary electrical leads (not shown) for actuating the perforating device 18.

Since it has been recently discovered that hard rock can be crumbled `with a laser beam, this method is readily adapted to carrying out the concepts of my invention. The substantially void space 14 can be anticipated from the detonation of a nuclear device; however, if necessary, sucient space for the perforating device 18 can be provided by some suitable form of under-reaming at the top of the permeable zone 11.

In some cases, the permeability of the central portion a of the rubble 15 will be so high as to effectively divert all or most of the fluids to be injected as will be discussed later. In such cases, an impermeable layer 22 is formed near the top of the central portion 15a of rubble 15. This is preferably accomplished by injecting some very viscose material such as a non-porous cement or resin formulation or other suitably selected agents which will solidify in situ within the rubble 15. The layer 22 tends to divert most of the injected fluids to the sides of the chimney, through the channels opened into the wall of the chimney (shown in dotted lines in FIGURE 1) and into the fractures 16 thus tending to improve the sweep eiciency in subsequent hydrocarbon recovery processes. Alternatively, the tendency for liuid injected near the top of the rubble 15 to flow preferentially through the central portion 15a is also reduced by injecting a fluid having a relatively high frictional resistance to flow, such as an emulsion or a foam. The use of foam injection to advance a combustion front down through a chimney is described in detail in copending application Ser. No. 689,- 181, tiled Dec. 8, 1967. Instead of or in addition to depositing a layer of impermeable material on or near the top of the central portion 15a of rubble 15, the central portion 15a of rubble 15 may be lled with a material having a relatively uniform permeability that is low compared with the average permeability of the fragmented oil shale. Such a material can comprise a bed of relatively ne sand, a layer of porous cement, or a porous mixture of resin and sand or the like.

Various means of inducing perforations or openings in the Wall 12a of the oil shale formation 12 surrounding the void space 14 may include explosives, jet or bullet perforators or systems for jetting a stream of abrasive fluid against the wall 12a of the shale oil formation 12.

As can be seen from the above discussion, the perforations into Wall 12a are preferably opened along generally radial directions from the central portion 15a of the rubble 15. This provides a radial extension of the substan-y tially solid-free void space 14 near the top of the permeable zone 11 and permits communication with the system of fractures lsurrounding the zone 11. Thus, as d iscussed below concerning FIGURES'Z and 3, when Huid is circulated through the permeable zone 11fror'n the top to bottom, full injection pressure is applied across the solid-free area thereby establishing better fluid communication withthe voutermost portions of thev fractured zone which includes the chimney-type permeable zone 11 and the fractured area surrounding zone 11. If the permeability within the fractured oil shale is sufficiently uniform, a so-injected uid will travel downward while tending to maintain a level horizontal front.

Various types of processes can be used to increase the volume of the permeable zone 11 created within the oil shale formation 12 utilizing the perforations in wall 12a of oil shale formation 12. FIGURES 2 and 3 illustrate alternate processes wherein like numerals refer to like parts of FIGURE 1.

In FIGURE 2, after the perforations have been made in the wall 12a of oil shale formation 12, well 17 is extended as at well portion 17a to the bottom of the rubble area 15 where communication is established with the rubble 15 by means of openings 23 formed in well portion 17a. Well 17 is preferably drilled into the bottom of rubble area 15 while the zone 11 is hot, or at least warm, from the explosion. Well 17 is equipped for injecting fluid through the borehole opening above packer 24 and through openings 25 formed in well 17 and into the upper portion of the permeable fragmented zone 11. The fluid circulates in a generally vertical downward ow path that includes flow through the perforations in the wall 12a of oil shale formation 12 surrounding permeable zone 11. The permeable channel providing unobstructed fluid communication between openings 25 in well 17 and openings 23 in well portion 17a Vis formed by known techniques. One preferred technique is the acidizing of a tuffaceous streak as described in application Ser. No. 619,259, filed Feb. 28, 1967. As is well known in the art, the fluid is heated prior to circulation by means of a heating device 31. In other words, the fluid is pumped by means of pump 26 through heating device 31, through the unobstructed permeable path as indicated by the arrows in FIGURE 2, and up tubing string 27 where the oil and gas entrained in the heated circulating fluid passes through a heat exchanger 28 and into separator 29. At this point, the oil and gas components are separated as is well known in the art. The uid is recirculated from separator 29 through pump 26 as again is well known in the art.

Further discussions concerning the circulation of a fluid until the oil shale derived fluidizable materials are entrained in the fluid can be found in copending Ser. No. 632,006, filed Apr. 19, 1967 to Prats.

Thus, it can be seen that the perforations in the wall 12a improve the flow of uid from the central portion 15a to the outlying portions of the permeable zone 11. This improves the eiciency of either an injection of fluid near the top and a production of uid near the bottom, or the reverse. An upward circulation can advantageously be used in a gas heating process as for example, a process ofthe type described in Patents Nos. 2,813,583; 3,233,- 668; 3,241,411 or the like. In these processes, the fluids circulating through the oil shale at the temperatures in the oil vshale are mainly gases which are circulated relatively rapidly in order to transport heat throughout the oil shale. A downflowing circulation can advantageously be used to advance a combustion front down through the oil shale or to permeate the oil shale with an oil solvent vapor or 'liquid'which is initially or tends to become, liquid due to either condensation orA dissolving of shale oil components within the chimney. A suitable extraction process of this type is described in copending application lSer. No. 656,815, led July v28, 1967.

FIGURE 3 shows an alternate process for increasing the volume of the permeable zone 12 created within oil shalev formation 12. vLike numerals refer to like elements of FIGURE 2. In place of the same'well being opened into both the void space 14 and the lower. end of the rubble area15, a well 30, independent of well 17, isopened into the lower end of the rubble 15. Thus, the circulating fluid exitsl fromy tubing string 27, is circulated through the perforations in the wall 12a of oil shale formation 12 and thus through fractures 16, and enters the well 30 at its lower end vadjacent to the bottom of lrubble 15. Again, conventional equipment and techniques, such as heating device 31, pump 26, separator 29 andV heat exchanger 28, can 'be used for pressurizing, heating, injecting, producing and separating components of the fluid that is circulated through the permeable zone 11.

I claim as my invention:

1. In a method of increasing the volume of the permeable zone created within a subterranean oil shale formation by detonating at least one relatively high energy explosive device within the formation to form a chimney of rubble surrounded by a fractured zone and having a substantially 'void space near its top, the improvement comprising the steps of opening la borehole into the substantially void space near the top of the chimney;

opening perforations into the wall of the fractured zone surrounding the upper end of the chimney of rubble adjacent the substantially void space along generally radial directions from the central portion of the rubble contained in the chimney;

establishing unobstructed fluid communication between the substantially void space and the lower end of the chimney of rubble; and

circulating fluid along the lines of unobstructed fluid communication established between the substantially void space and the lower end of the chimney along a generally vertical path that includes flow through at least some of the perforations into the fractured zone surrounding the upper end of the chimney.

2. The method of claim 1 including the step of depositing a layer of permeability reducing material along the upper portion of the rubble adjacent the center of the chimney containing the more permeable fragmented oil shale.

3. The method of claim 2 including the step of impregnating the rubble adjacent the center of the chimney containing the fragmented oil shale with material having a relatively uniform permeability that is low compared with the average permeability of the rubble.

4. The method of claim 1 wherein the step of circulating fluid includes injecting a fluid having a relatively high frictional resistance to flow through the rubble contained in the chimney.

5. The method of claim 1 wherein the step of establishing unobstructed fluid communication between the substantially void space and the lower end of the chimney of rubble includes opening a borehole into the rubble contained in the lower end of the chimney.

6. The method of claim 1 wherein the step of establishing unobstructed fluid communication between the substantially void space and the lower end of the chimney of rubble includes opening the same borehole into both the substantially void space and the lower end of the Chimney.

7. The method of claim 1 wherein the step of opening perforations includes jetting a stream of abrasive fluid against the wall of the fractured zone until openings are formed therein.

8. The method of claim 1 wherein the step of opening perforations includes the steps of:

lowering a perforating device through the borehole opening; and

moving the perforating device into operative position within the substantially void space of the chimney.

9. The method of claim 1 including the step of extracting shale oil from the circulating fluid.

10. The method of claim 1 including the step of irnpregnating the rubble adjacent the center of the chimney containing the fragmented oil shale with material having a realtively uniform permeability that is low compared with the average permeability of the rubble.

References Cited UNITED STATES PATENTS 2,813,583 11/1957 Marx et al. 166-40 X 3,113,620 12/1963 Hemminger 166-11 3,180,414 4/1965 Parker 166-40 X 3,273,640 9/1966 Huntington 166-40 X 3,285,335 11/1966 Reistle 166-40 X 3,303,881 2/1967 Dixon 166--36 3,342,257 9/1967 Jacobs et al 166-36 STEPHEN J. NOVOSAD, Primary Examiner U.S. Cl. X.R. 166-247, 297, 303 

